A useful method of demonstrating this is the laundry analogy.

Let's say that there are four loads of dirty laundry that need to be washed, dried, and folded.

We could do one load at a time by doing the following

1. Wash for 30 minutes
2. Dry for 40 minutes
3. Then take 20 minutes to fold the clothes
4. Then do the second, third, and fourth the same way

Supposing we started at 6 PM and worked as efficiently as possible, we would still be doing laundry until midnight as seen in the picture.